Thursday, 13 May 2010

Adventures in beer design (Part II)…..do you realise the gravity of the situation?

I was looking at some old brewing data for Scotch Ales and was somewhat alarmed at the gravity of the beers. I mean this in the brewers sense of the word gravity rather than as in seriousness. However, these guys brewed at seriously high gravities.

I suppose I should try and explain the importance of gravity to brewing. Gravity, or more specifically, specific gravity, is a measure of the density of a liquid in comparison to the density of water. Brewers use this to measure the concentration of sugar in their worts. So what? Well…if you can measure the concentration of sugar then you have a fair idea how much alcohol you produce. You see yeast turns sugar into alcohol and as the sugar is turned into alcohol the gravity of the liquid falls. There are two main gravity measurements that brewers use. The first is Original Gravity, that is the concentration of sugar before the yeast goes into action. The second is Final Gravity, that is the concentration of sugar after the yeast has finished fermenting. From these two figures we can estimate the amount of alcohol produced using a formula that someone cleverer than me made up.

(Original Gravity – Final Gravity) x 0.129 = % Alcohol by Vol (ABV)

Here’s an example for a Scotch Ale from my old friend W H Roberts. This is a 19th century 140/- Scotch Ale.

Original Gravity = 1130.75Final Gravity = 1061Therefore the calculated ABV is a whopping 8.99%!! According to the literature the actual ABV was 8.86% so not a million miles away.

The other thing to note in this beer is the level of attenuation. What’s attenuation I hear you ask? Attenuation is the degree to which the yeast has managed to convert sugars to alcohol. Why is it important? (I wish you’d stop asking awkward questions). It’s important because it determines the amount of residual (unfermented) sugar remaining in the beer. This will affect the sweetness and the mouthfeel of the beer. A large amount of unfermented sugar (low attenuation) will result in a sweet tasting beer with a full mouthfeel. Conversely a beer that has been attenuated to high degree may be thin and unbalanced. The beer above has a very high level of unfermented sugar in it – not only did it start off with a lot of sugar in the first place (OG of 1130) but the level of attenuation was only 53.36% i.e. just over half the sugar was fermented. Attenuation is a funny thing and without going into the details there are two possible methods of determining it. One is called apparent attenuation and the other is called real attenuation. Let me know if you want to know more!!

Ok, so much for a 19th century Scotch Ale – high in alcohol and high in unfermented sugar, probably not to today’s drinker’s taste but hopefully it has given you an idea of how Original Gravity, Final Gravity and Alcohol content link together.